Rubber products are frequently made up of several rubber layers each with a same or a different chemical composition. During this “build up,” the rubber layers must adhere to one another adequately in their pre-vulcanized state. For example, an assembled tire blank is required to hold together for a fairly long period prior to vulcanization. It is therefore important that the rubber mixtures used have an adequate “tack,” the force required to pull apart two pre-vulcanized rubber mixtures which have been pressed together under certain defined conditions. While natural rubber mixtures generally have good tackiness, mixtures of synthetic rubbers are much less tacky and, in extreme cases, possess no tackiness at all. Therefore, it has been common practice to add a tackifier to less tacky rubbers or rubber mixtures to increase their tack. In synthetic rubber products, synthetic rubber adhesive compositions are employed to improve tack and provide good cured adhesion. Moreover, the rubber composition must not only have good initial tack, but also remain sufficiently tacky during the manufacturing process (i.e., good tack retention), even when the process is interrupted for fairly long periods, which is not unusual, particularly when manufacturing involves processes at different locations or requires storage and/or transport of pre-finished goods.
To ensure adequate tack, conventional methods of preparing alkylphenol-aldehyde resins typically use high-purity alkylphenols (e.g., commercially available resin-grade alkylphenols having a purity level of higher than about 98 wt %). However, using a purified alkylphenol is expensive and can significantly increased processing time and manufacturing costs.
Therefore, there remains a need to develop tackifiers which provide increased tack and tack retention, and, at the same time, also provide a cost-effective and time-efficient solution. A particular need exists in the tire industry because of the poor tack and tack retention of synthetic rubber compositions, such as commercial SBR-based tire compositions. This invention answers that need.